This invention relates generally to the field of integrated circuit interconnections, and more particularly to a bumped connection structure used on integrated circuits, and on other assemblies, such as integrated circuit packages.
A number of forces have influenced the selection o structures which provide connection of an integrateded circuit to the next level of interconnection. Lead density has been a major factor in focusing designers toward connections which are contained within the area of the integrated circuit or of first level packaging, such as flip-chip, ball grid array (BGA) and chip size packages (CSP). These interconnections are generally referred to as area array interconnections and are formed by a bump type of structure as opposed to a flexible lead type of structure which extends out from the perimeter of the integrated circuit element.
Connections for these assemblies are designed as a finely pitched matrix of conductive contacts on the surface with a corresponding matrix of contact pads on the next level of interconnection. Typically the connecting structures have included solder balls or bumps of various lead (Pb) and tin (Sn)compositions, which are reflowed to form the electrical and mechanical connection.
Flip-chip is the oldest of these technologies wherein the input/output contact pads on the surface of the integrated circuit device are soldered directly to the corresponding contact pads of a substrate. The original flip-chip concept employed small solder coated copper balls sandwiched between the chip termination lands and the contact pads of the ceramic substrate. Owing to manufacturing difficulty in handling and placement of the balls, this procedure was replaced by forming solder connection structures or bumps on the chip terminals while still in wafer form. This consisted of depositing thin films of metal on the wafer and patterning the contact pads by photolithography and etch processing and forming bumps on said contact pads by evaporating solder through apertures in metal masks. The wafers were diced, the chips aligned to the substrate and solder reflowed. Owing to the short, rigid connecting structures, fatigue at the solder joints is a concern. Waste treatment for clean-up of lead containing solder from said metal masks is both expensive and a growing environmental issue. Alternately, solder bumps and/or solder over copper bumps have been formed on the contact pads by combinations of sputtered and electroplating.
More recently, techniques have been described for forming flip-chip connecting structures by capturing prefabricated spheres of solder in a matrix of tacky areas defined by photosensitive polymers, aligning the matrix which corresponds to the contact pads on the chips in wafer form and releasing by reflowing the solder.
Flip-chip interconnections have continued to evolve because the technology provides advantages for maximum lead density, and very low inductance. However, the connecting structures have been largely restricted to the use of lead/tin solders of different compositions which are subject to fatigue failures as a function of thermally induced stresses. Further, lead (Pb) containing solders on the surface of soft error sensitive devices has caused concern due to alpha particle emission from lead (Pb), as well as environmental issues with the use of lead (Pb).
Ball grid array (BGA) packages have gained acceptance as low cost, high yielding packages which offer the maximum in board space efficiency as a result of direct connections under the package to the printed wiring board. Both plastic and ceramic BGA packages have been commercialized. Connections between the integrated circuit and the top surface of the BGA package can be by wire bonding or by flip-chip connection. The connecting structures on the bottom surface of the BGA package are typically solder bumps configured in an area array. Typically the structures are formed by partially reflowing eutectic or other lead/tin solders to metal contact pads on the package and these in turn are attached to the printed wiring board by reflow using a solder paste which has been screened onto contact pads on the board. Inspection of the joints has been a concern for BGA packages assembled to boards. The reliability concerns are somewhat similar to those for flip-chip in that thermally induced stresses in the solder joints result from mismatches CTE and Young""s modulus and which may vary within the package and board area due to the large size and underlying construction. In addition, stand-off height must be controlled, both to minimize stresses on the solder joints and to allow cleaning. Package weight can contribute to the difficulty in controlling stand off distance of solder bumped packages.
Chip scale (CSP) packaging provides the minimum size at no more than 1.2 times that of the integrated circuit, and is a directly surface mountable package which facilitates testing and ease of handling. There are a number of package styles available, both with leads and bump connection structures. The bumped structures, typically Sn/Pb solder bumps are attached by reflow of prefabricated spheres onto a contact pad on the CSP. As with BGA and flip-chip, the presence of leaded compounds presents an environmental issue to some users. And in particular, thermal mismatch must be compensated in order to avoid solder fatigue failures because the package is dominated by rigid, low CTE silicon in close proximity to a printed wiring board. Clean-up of fluxes presents reliability concerns with the small stand-off height of CSP.
For some specific applications where assembly temperature is restricted by the circuit being assembled, such as liquid crystal displays, raised polymeric structures have been formed on contact pads. The exposed surface of the structure is coated with a metallic film, and attached to the corresponding pads by solder reflow or by conducting adhesive materials. This process of fabricating polymeric bumps is extremely expensive, is limited by processing capabilities and in some cases will not be compatible with thermal testing routinely required of integrated circuits.
Similarly, composite bump structures of metal bumps with tin or other metallic coatings have been formed on wafers or on polymeric flex films for tape automated bonding (TAB) connections. The fabrication process is not unlike that described above for polymeric bumps and is expensive.
The primary object of this present invention is to provide an integrated circuit, or circuit assemblies with bump connections which allow attachment to the next level of interconnection by using prefabricated structures coated with solder-compatible metals, and which are attached to the circuit or circuit package by solder. The integrated circuit structures compatible with the present invention are those which employ bump or ball connections typical of area array assembly, namely but not limited to ball grid array packages, flip-chip assembly and chip scale packages.
The composite bump structures comprise two or more thin films of solder compatible metal or metals coated onto a prefabricated structure and connecting said structure to the contact pads of an integrated circuit element by solder. The prefabricated core element is comprised of metal, ceramic or polymer and is of the approximate size and uniformity as specified by the integrated circuit element. The connecting structure will be compatible with known manufacturing techniques for attachment to the next level of interconnection. This device provides an economical and reliable connection and does not have the disadvantages of devices assembled by the techniques described above.
Another object of the invention is to provide a solderable composite prefabricated connecting structure whose thermal coefficients of expansion and Young""s modulus are selected to improve reliability.
Still another object of the invention is to provide a solderable composite prefabricated bump structure whose thermal conductivity is optimized to improve performance and reliability of the circuit.
Further, it is an object of the invention is to provide a solderable composite prefabricated connecting structure which provides controlled separation distance or stand-off height between the integrated circuit element and the next level of interconnection.
Another object of the invention is to provide a solderable composite prefabricated connecting structure whose shape is selected to improve reliability.
Another object of the invention is to provide an integrated circuit connection structure with metallurgical composition of the coated metal layers whose surface is optimized for compatibility with solders, and the underlying layers of sufficient thickness to provide electrical contact.
Yet another object of the invention is to provide an integrated circuit connection structure whereby the attachment temperature to contact pads of one element is significantly different from the attachment temperature at the corresponding pads of the next surface. This feature facilitates ease of rework.
Another object of the invention is to provide an integrated circuit connection of solderable composite prefabricated bumps which is lead (Pb) free, and uses reliable solder attachment techniques which are commercially available.
Another object of the invention is to provide a solderable composite connecting structure which is lower in alpha particle emission than lead (Pb) bearing solders.
Yet another object of the invention is to provide a solderable composite connection structure which is amenable to inspection by x-ray.
A further object of the invention is to provide an integrated circuit element which weighs less than said elements constructed with lead (Pb) bearing connections.
In accordance with the present invention, there is provided a flexible method of forming the mechanical and electrical connection between an integrated circuit element and connecting structure. This technique is applicable to integrated circuit elements such as BGA, flip-chip, CSP, or other assemblies which make use of bump connection structures. The method described provides for using materials which are optimized to the type of integrated circuit element being assembled; i.e. BGA, flip-chip, or CSP. Further, the invention comprises a method to transfer composite bump structures to the integrated circuit contact pads and is also applicable to each of said package types. Contact pads of said elements are coated with solder paste through apertures in a stencil and composite bumps aligned, brought into contact and the assembly heated to form a metallurgical bond. The transfer method comprises forming an array of patterned areas which register to the location of contact pads. One bump is captured per pattern area and retained until the bumps are aligned to the receiving pads. The preferred method for forming the patterned arrays provides a photoimagable adhesive coated on a transparent carrier film. Upon heating the adhesive loses tackiness and the bumps are transferred to the contact pads with solder paste. Heating is accomplished by infrared, convection or many other techniques commercially available solder reflow processes, and preferably is by radiation heating of the substrate. Equipment designed for aligning, contacting and heating has been disclosed previously in a related disclosures from Texas. Instruments, Inc. in Docket No. TI 26059P and TI-23158P and these are incorporated herein by reference. Further, a suitable film with photosensitive adhesive to form tacky patterned areas is available from E.I. duPont de Nemours and Company. U.S. Pat. No. 5,356,751 is incorporated herein by reference. The integrated circuit or package with connecting structure is used in conjunction with directly soldering said structure to the next level interconnection.
Other objects and advantages of the present invention will become apparent from the following descriptions, taken in connection with the accompanying drawings, wherein, by way of illustration and example, an embodiment of the present invention is disclosed.
An integrated circuit or package with bumped composite connection structure and methods of forming the said structure comprising: a prefabricated core structure of composition, size and shape to support the design requirements of said integrated circuit or package, a coating layer of metal or metals which are electrically conductive and compatible with solder, and solder reflow connection of said structure to the integrated circuit or package.
Said coating metal(s) will be of composition and sufficient thickness for the connecting structure to provide electrical contact between the integrated circuit or package and the next level of interconnection. The coating will be metallurgically compatible with solders. Further, said coating will be adhered to and completely surround the core structure.
Composition of said prefabricated core will be optimized to meet the requirements of the integrated circuit or package type; i.e., a metal core for maximum thermal dissipation, whereas a polymeric or ceramic core material will be optimized for thermal-mechanical requirements by plastic or ceramic substrates. Said connecting structure is preferably free of lead (Pb) containing materials.
A method of forming a connection structure on an integrated circuit or package comprising the steps of providing a prefabricated bump structure designed with conductive, solderable metal layers completely surrounding a core structure, and providing a solder attachment, preferably lead (Pb) free to the integrated circuit or package. Techniques for alignment and attachment of the composite bump structure are compatible with existing manufacturing techniques and the preferred method is based on previously and referenced disclosures.
The drawings constitute a part of this specification and include exemplary embodiments to the invention which may be embodied in various forms.